Method of and apparatus for image processing

ABSTRACT

A monochromatic image signal is made from a color image signal. Values of brightness of pixels in a color border region of a type where two color regions which are small in brightness difference and large in chromaticity difference are positioned side by side are changed to new values different from those represented by the image signal component for the pixel of the color image signal, and a monochromatic image signal is generated with values of brightness of pixels in the color border region set to the new values and values of brightness of pixels in regions other than the color border region set to those as represented by the image signal components for the pixels of the color image signal.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a method of and an apparatus for imageprocessing which are applicable to an image display, a copier, afacsimile and the like, and more particularly to a method of and anapparatus for image processing in which a color image signal (made up ofimage signal components representing image data on respective pixels) isconverted into a monochromatic (black and white) image signal and amonochromatic image output is obtained on the basis of the monochromaticimage signal.

[0003] 2. Description of the Related Art

[0004] Recently as the color scanner becomes less expensive and/or bythe advent of a digital camera and the like, it becomes feasible toeasily make a color image signal by the use of various image input unit,and accordingly, a chance for the display, copier, facsimile or the liketo output a monochromatic image on the basis of a color image signal hasbeen increased.

[0005] When a monochromatic image is to be output on the basis of acolor image signal, the color image signal has to be once converted to amonochromatic image signal by means of an image processing apparatus asdisclosed, for instance, in Japanese Unexamined Patent Publication No.6(1994)-59657.

[0006] In the method of conversion disclosed in Japanese UnexaminedPatent Publication No. 6(1994)-59657, the color image signal is dividedinto a hue signal, a brightness signal and a chroma signal and a tonevalue for a pixel is determined taking into account the hue of the pixelin addition to the brightness of the pixel according to gray tables(tone conversion tables) which have been prepared hue by hue taking intoaccount the fact that colors at the same brightness can be vieweddifferent in brightness when hues are different.

[0007] For example, when a color image signal component L*_(in) for apixel is input as shown in FIG. 8A, a monochromatic image signalcomponent L*_(Rout) is output according to a gray table for red LUT_(R)when the hue represented by the color image signal component L*_(in) isred, and a monochromatic image signal component L*_(Gout) is outputaccording to a gray table for green LUT_(G) when the hue represented bythe color image signal component L*_(in) is green. As a result, when tworegions which are of the same brightness and of different hues existside by side on a color image, the two regions are expressed differentin brightness on a monochromatic image, whereby the difference in hue ona color image can be expressed on a monochromatic image, and informationon brightness which can be visually detected on the color image can bepreserved on the monochromatic image. That is, halftone image signalcomponents of a color image signal can be clearly expressed bymonochromatic image components.

[0008] However, the method of conversion disclosed in JapaneseUnexamined Patent Publication No. 6(1994)-59657 is disadvantageous inthat the monochromatic image signal component L*_(Gout) according to agray table for green LUT_(G) for a green color image signal componentL*_(Gin) and the monochromatic image signal component L*_(Rout)according to a gray table for red LUT_(R) for a red color image signalcomponent L*_(Rin) representing brightness different from that of thegreen color image signal component L*_(Gin) can become equal to eachother as shown in FIG. 8B. That is, pixels which are different from eachother in both hue and brightness on a color image can be expressed inthe same tone on a monochromatic image, whereby information on adifference in hue between two adjacent regions is lost on amonochromatic image.

SUMMARY OF THE INVENTION

[0009] In view of the foregoing observations and description, theprimary object of the present invention is to provide an imageprocessing method and an image processing apparatus which can make amonochromatic image signal from a color image signal without loss ofbrightness information and hue information.

[0010] In accordance with the present invention, the values ofbrightness of pixels in a color border region of a type where two colorregions which are small in brightness difference and large inchromaticity difference are positioned side by side are changed to thosedifferent from those represented by the color image signal and thevalues of brightness of pixels in regions other than the color borderregion are set to those as represented by the color image signal as theyare, whereby the color border region can be enhanced on a monochromaticimage as change in tone.

[0011] That is, in accordance with a first aspect of the presentinvention, there is provided a method of making a monochromatic imagesignal made up of image signal components representing image data onrespective pixels from a color image signal made up of image signalcomponents representing image data on respective pixels, the methodcharacterized in that

[0012] values of brightness of pixels in a color border region of a typewhere two color regions which are small in brightness difference andlarge in chromaticity difference are positioned side by side are changedto new values different from those represented by the image signalcomponent for the pixel of the color image signal, and

[0013] a monochromatic image signal is generated with values ofbrightness of pixels in the color border region set to said new valuesand values of brightness of pixels in regions other than the colorborder region set to those as represented by the image signal componentsfor the pixels of the color image signal.

[0014] The color image signal may be, for instance, an image signalobtained by reading a color original by an image scanner or an imagesignal obtained by a digital camera.

[0015] The values of brightness and chromaticity may be in any terms.For example, the values of brightness and chromaticity may berepresented by L*a*b* values or L*u*v* values in a CIE color system.

[0016] The monochromatic image signal obtained may be used in any way.For example, the monochromatic image signal may be binary-coded and usedfor printing or making a stencil, or may be used for displaying a colorimage on a monochromatic display, or may be used for transferring acolor original from a color facsimile to a black and white facsimile.

[0017] In a first aspect of the method of the present invention, animage processing method comprises

[0018] the step of determining whether a designated pixel is a pixel ina color border region of a type where two color regions which are smallin brightness difference and large in chromaticity difference arepositioned side by side on the basis of the difference in value ofbrightness between the designated pixel and neighbor pixels near thedesignated pixel and the difference in value of chromaticity between thedesignated pixel and the neighbor pixels,

[0019] the steps of, when the designated pixel is determined to be apixel in a color border region of said type, obtaining the values ofbrightness and chromaticity of at least two colors representative ofcolors (will be sometimes referred to as “the representative colors”)forming the color border on the basis of the values of brightness andchromaticity of the neighbor pixels, and changing the value ofbrightness of the designated pixel to a new value different from thatrepresented by the image signal component for the pixel of the colorimage signal on the basis of the values of brightness and chromaticityof the designated pixel as represented by the image signal component forthe pixel of the color image signal, the values of brightness andchromaticity of the respective representative colors and a preset orderof priority of colors, and

[0020] the step of making a monochromatic image signal by setting valuesof brightness of pixels in the color border region to said new valuesand setting values of brightness of pixels in regions other than thecolor border region to those as represented by the image signalcomponents for the pixels of the color image signal. That is, in theimage processing method of the first aspect, whether a designated pixelis a pixel in a color border region of said type is determined on thebasis of the values of the designated pixel and the neighbor pixels, andthe value of brightness of the designated pixel in the monochromaticimage signal is changed from that represented by the color image signalcomponent when the designated pixel is determined to be a pixel in acolor border region of said type.

[0021] The colors representative of colors forming the color border arethe typical colors to which two colors on opposite sides of the colorborder belong and may be determined in any known method.

[0022] The range of the neighbor pixels may be changed according to thetype of image to be processed so that color border portions can beaccurately determined. For example, in the case of an image ofillustration type, the range of the neighbor pixels may be of arelatively small size (e.g., 3×3 pixels). As the size of the range ofthe neighbor pixels becomes smaller, the amount data to be handled isreduced and the processing is facilitated.

[0023] In the method of the first aspect, when the designated pixel isdetermined to be a pixel in a color border region of said type, thevalue of brightness of each pixel in a predetermined region about thedesignated pixel may be changed to a value different from thatrepresented by the image signal component for the pixel of the colorimage signal on the basis of the values of brightness and chromaticityof the pixel, the values of brightness and chromaticity of therespective representative colors and a preset order of priority ofcolors.

[0024] In this case, the value of brightness is changed for not only thepixel determined to be a color border pixel but also the pixels in theregion about the color border pixel. That is, the value of brightness ischanged region by region.

[0025] The size of the predetermined region about the designated pixelmay be set so that the degree of enhancement of the color border regioncan be visually recognized, and may be equal to, smaller than or largerthan the size of the range of the neighbor pixels. That is, as describedabove, the size of the range of the neighbor pixels may be set from theviewpoint of accuracy in determining color border regions and the sizeof the predetermined region about the designated pixel may be set fromthe viewpoint of easiness in visually recognizing the degree ofenhancement of the color border regions. For example, the size of therange of the neighbor pixels may be set to 3×3 with the size of thepredetermined region about the designated pixel set to 3×3 or 5×5. Asthe pixel density increases, the area of the predetermined region on theoutput image becomes smaller for a given size of the predeterminedregion. Accordingly, it is preferred that the size of the predeterminedregion be increased as the pixel density increases in order tofacilitate visual recognition of the degree of enhancement of the colorborder regions.

[0026] In a second aspect of the method of the present invention, animage processing method comprises

[0027] the steps of dividing the color image signal into a plurality ofblocks of a predetermined size, and determining the values of brightnessand chromaticity of each block on the basis of the values of brightnessand chromaticity of each pixel in the block represented by image signalcomponents for the pixel of the color image signal,

[0028] the step of determining whether a designated block is a block ina color border region of a type where two color regions which are smallin brightness difference and large in chromaticity difference arepositioned side by side on the basis of the difference in value ofbrightness between the designated block and neighbor blocks near thedesignated block and the difference in value of chromaticity between thedesignated block and the neighbor blocks,

[0029] the steps of, when the designated block is determined to be ablock in a color border region of said type, obtaining the values ofbrightness and chromaticity of at least two colors representative ofcolors forming the color border on the basis of the values of brightnessand chromaticity of the neighbor blocks, and changing the value ofbrightness of each pixel in the designated block to a new valuedifferent from that represented by the image signal component for thepixel of the color image signal on the basis of the values of brightnessand chromaticity of the pixel, the values of brightness and chromaticityof the respective representative colors and a preset order of priorityof colors, and

[0030] the step of making a monochromatic image by setting values ofbrightness of pixels in the block in the color border region to said newvalues and setting values of brightness of pixels in the blocks inregions other than the color border region to those as represented bythe image signal components for the pixels of the color image signal.

[0031] In accordance with a second aspect of the present invention,there is provided an apparatus for making a monochromatic image signalmade up of image signal components representing image data on respectivepixels from a color image signal made up of image signal componentsrepresenting image data on respective pixels, the apparatuscharacterized by having

[0032] a color border enhancement processing means which changes valuesof brightness of pixels in a color border region of a type where twocolor regions which are small in brightness difference and large inchromaticity difference are positioned side by side to new valuesdifferent from those represented by the image signal components for thepixels of the color image signal and

[0033] a monochromatic image signal generating means which makes amonochromatic image signal by setting values of brightness of pixels inthe color border region to said new values and setting values ofbrightness of pixels in regions other than the color border region tothose as represented by the image signal components for the pixels ofthe color image signal.

[0034] In a first aspect of the apparatus of the present invention,which is for carrying out the first aspect of the method of the presentinvention, an image processing apparatus comprises

[0035] a color border determining means which determines whether adesignated pixel is a pixel in a color border region of a type where twocolor regions which are small in brightness difference and large inchromaticity difference are positioned side by side on the basis of thedifference in value of brightness between the designated pixel andneighbor pixels near the designated pixel and the difference in value ofchromaticity between the designated pixel and the neighbor pixels,

[0036] a representative color determining means which obtains the valuesof brightness and chromaticity of at least two colors representative ofcolors forming the color border concerning the designated pixeldetermined to be a pixel in the color border region on the basis of thevalues of brightness and chromaticity of the neighbor pixels,

[0037] a color border enhancing brightness calculating means whichchanges the value of brightness of the designated pixel determined to bea pixel in the color border region to a new value different from thatrepresented by the image signal component for the pixel of the colorimage signal on the basis of the values of brightness and chromaticityof the designated pixel as represented by the image signal component forthe pixel of the color image signal, the values of brightness andchromaticity of the respective representative colors and a preset orderof priority of colors, and

[0038] a monochromatic image signal generating means which generates amonochromatic image signal by setting values of brightness of pixels inthe color border region to said new values and setting values ofbrightness of pixels in regions other than the color border region tothose as represented by the image signal components for the pixels ofthe color image signal.

[0039] In the apparatus of the first aspect, the color border enhancingbrightness calculating means may be arranged to change the value ofbrightness of each pixel in a predetermined region about the designatedpixel to a new value different from that represented by the image signalcomponent for the pixel of the color image signal on the basis of thevalues of brightness and chromaticity of the pixel, the values ofbrightness and chromaticity of the respective representative colors anda preset order of priority of colors when the designated pixel isdetermined to be a pixel in the color border region of said type, withthe monochromatic image signal determining means arranged to generate amonochromatic image signal by setting values of brightness of pixels inthe predetermined region to said new values.

[0040] For example, the color border determining means may be arrangedto determine that the designated pixel is a pixel in a color borderregion of said type when there is at least one pixel in the neighborpixels whose difference in brightness from that of the designated pixelis smaller than a first threshold value and whose difference inchromaticity from that of the designated pixel is larger than a secondthreshold value.

[0041] For example, the representative color determining means maycalculate the differences in chromaticity between a pixel in theneighbor pixels whose difference in brightness from that of thedesignated pixel is smaller than a first threshold value and whosedifference in chromaticity from that of the designated pixel is largerthan a second threshold value and the other pixels in the neighborpixels, and take as the values of chromaticity of the representativecolors the values of chromaticity of a pair of pixels which give amaximum difference in chromaticity.

[0042] Further, the color border enhancing brightness calculating meansmay be arranged to determine which of the representative colors thecolor of the designated pixel belongs to on the basis of the values ofbrightness and chromaticity of the designated pixel and the values ofbrightness and chromaticity of each of the representative colors, tochange the value of brightness of the designated pixel to a new value byadding a predetermined constant to that represented by the image signalcomponent for the pixel of the color image signal when therepresentative color to which the color of the designated pixel belongsis higher in the order of priority of colors than the otherrepresentative color, and to change the value of brightness of thedesignated pixel to a new value by subtracting a predetermined constantfrom that represented by the image signal component for the pixel of thecolor image signal when the representative color to which the color ofthe designated pixel belongs is lower in the order of priority of colorsthan the other representative color.

[0043] Further, the color border enhancing brightness calculating meansmay be arranged to determine which of the representative colors thecolor of each pixel in a predetermined region belongs to on the basis ofthe values of brightness and chromaticity of the pixel and the values ofbrightness and chromaticity of each of the representative colors, tochange the value of brightness of the pixel to a new value by adding apredetermined constant to that represented by the image signal componentfor the pixel of the color image signal when the representative color towhich the color of the pixel belongs is higher in the order of priorityof colors than the other representative color, and to change the valueof brightness of the designated pixel to a new value by subtracting apredetermined constant from that represented by the image signalcomponent for the pixel of the color image signal when therepresentative color to which the color of the pixel belongs is lower inthe order of priority of colors than the other representative color.

[0044] In a second aspect of the apparatus of the present invention,which is for carrying out the second aspect of the method of the presentinvention, an image processing apparatus comprises

[0045] a block brightness/chromaticity calculating means which dividesthe color image signal into a plurality of blocks of a predeterminedsize and determines the values of brightness and chromaticity of eachblock on the basis of the values of brightness and chromaticity of eachpixel in the block represented by image signal components for the pixelof the color image signal,

[0046] a color border determining means which determines whether adesignated block is a block in a color border region of a type where twocolor regions which are small in brightness difference and large inchromaticity difference are positioned side by side on the basis of thedifference in value of brightness between the designated block andneighbor blocks near the designated block and the difference in value ofchromaticity between the designated block and the neighbor blocks,

[0047] a representative color determining means which obtains the valuesof brightness and chromaticity of at least two colors representative ofcolors forming the color border concerning the designated blockdetermined to be a block in the color border region on the basis of thevalues of brightness and chromaticity of the neighbor blocks,

[0048] a color border enhancing brightness calculating means whichchanges the value of brightness of each of the pixels in the blockdetermined to be a block in the color border region to a new valuedifferent from that represented by the image signal component for thepixel of the color image signal on the basis of the values of brightnessand chromaticity of the pixel as represented by the image signalcomponent for the pixel of the color image signal, the values ofbrightness and chromaticity of the respective representative colors anda preset order of priority of colors, and

[0049] a monochromatic image signal generating means which generates amonochromatic image signal by setting values of brightness of pixels inthe color border region to said new values and setting values ofbrightness of pixels in regions other than the color border region tothose as represented by the image signal components for the pixels ofthe color image signal.

[0050] For example, the color border determining means may be arrangedto determine that the designated block is a block in a color borderregion of said type when there is at least one block in the neighborblocks whose difference in brightness from that of the designated blockis smaller than a first threshold value and whose difference inchromaticity from that of the designated block is larger than a secondthreshold value.

[0051] For example, the representative color determining means maycalculate the differences in chromaticity between a block in theneighbor blocks whose difference in brightness from that of thedesignated block is smaller than a first threshold value and whosedifference in chromaticity from that of the designated pixel is largerthan a second threshold value and the other blocks in the neighborblocks, and take as the values of chromaticity of the representativecolors the values of chromaticity of a pair of blocks which give amaximum difference in chromaticity.

[0052] Further, the color border enhancing brightness calculating meansmay be arranged to determine which of the representative colors thecolor of each pixel in the block determined to be a block in a colorborder region belongs to on the basis of the values of brightness andchromaticity of the pixel and the values of brightness and chromaticityof each of the representative colors, to change the value of brightnessof the pixel to a new value by adding a predetermined constant to thatrepresented by the image signal component for the pixel of the colorimage signal when the representative color to which the color of thepixel belongs is higher in the order of priority of colors than theother representative color, and to change the value of brightness of thepixel to an value by subtracting a predetermined constant from thatrepresented by the image signal component for the pixel of the colorimage signal when the representative color to which the color of thepixel belongs is lower in the order of priority of colors than the otherrepresentative color.

[0053] In the present invention, the order of priority of colors may bein the order of white (W), yellow (Y), cyan (C), magenta (M), red (R),green (G), blue (B), black (K) or may be in the order of the values ofbrightness of the representative colors.

[0054] In accordance with the present invention, since pixels in a colorborder region of a type where two color regions which are small inbrightness difference and large in chromaticity difference arepositioned side by side are given values of brightness different fromthose represented by the color image signal and pixels in regions otherthan such a color border region are given the same values of brightnessas represented by the color image signal, the color border region can beenhanced so as to be able to be recognized as change in tone in themonochromatic image while tone information on the region other than thecolor border region on the color image can be held even in themonochromatic image.

[0055] Accordingly, a color image can be converted into a monochromaticimage without loss of information on brightness and hue irrespective ofbrightness and hue on the color image, and at the same time, the problemthat the regions which are different from each other in both hue andbrightness on the color image are expressed in the same tone on themonochromatic image can be avoided.

[0056] When the image input apparatus such as an image scanner or theimage output apparatus such as stencil making apparatus is relativelylow in resolution, the method in accordance with the first aspect andthe apparatus for carrying out the method in accordance with the firstaspect in which the values of brightness are changed only for pixelswhich are determined to be a pixel in the color border region andaccordingly the color border is enhanced by pixel are suitable.

[0057] To the contrast, when the image input apparatus or the imageoutput apparatus is relatively high in resolution, the effect ofenhancement of the color border region is difficult to recognize whenthe color border region is enhanced by pixel. In such a case, the methodin accordance with the second aspect and the apparatus for carrying outthe method in accordance with the second aspect in which the values ofbrightness are changed for pixels which are determined to be a pixel inthe color border region and for the neighbor pixels and accordingly thecolor border is enhanced by block are suitable.

[0058] When the color image signal represents an image such as of acolor halftone original formed by superposing three colors, C, M and Y,where color frequently changes depending on the place, a color borderother than that visually recognized by humans can be detected if thecolor border region is determined pixel by pixel, which can result inundesirable brightness enhancement. In such a case, by determining thecolor border region block by block as in the method of the second aspectof the present invention, the color border region can be determined andbrightness can be enhanced so as to conform to human view.

[0059] The method in accordance with the second aspect and the apparatusfor carrying out the method in accordance with the second aspect inwhich the values of brightness are changed for all the pixels in theblock may be applied irrespective of whether the image input apparatusand/or the image output apparatus are low or high in resolution.

[0060] By obtaining the values of brightness and chromaticity of thecolors representative of colors forming the color border on the basis ofthe values of brightness and chromaticity of the neighbor pixels/blocks,and obtaining the value of color border enhancing brightness of eachpixel on the basis of the values of brightness and chromaticity of thepixel/block, the values of brightness and chromaticity of the respectiverepresentative colors and a preset order of priority of colors, thelevel of enhancement can be set on the basis of general conditionsaround the color border region and an excessive enhancement can beavoided.

[0061] The pixel/block in the color border region and/or therepresentative colors may be determined in various ways and thebrightness enhancement in the color border region may be effected invarious ways.

BRIEF DESCRIPTION OF THE DRAWINGS

[0062]FIG. 1 is a block diagram showing an image processing apparatusfor carrying out an image processing method in accordance with a firstor second aspect of the present invention,

[0063]FIG. 2A is a view for illustrating a designated pixel andreference pixels in the method of the first embodiment of the presentinvention,

[0064]FIG. 2B a view for illustrating color border enhancementprocessing in the method of the first embodiment of the presentinvention,

[0065]FIG. 3 is a view showing an example of a monochromatic viewobtained by enhancing a color border region of a color image by themethod of the first embodiment,

[0066]FIG. 4 is a view showing an a* -b* plane for determining therepresentative colors,

[0067]FIG. 5A is a view for illustrating a designated pixel andreference pixels in the method of the second embodiment of the presentinvention,

[0068]FIG. 5B is a view for illustrating color border enhancementprocessing in the method of the second embodiment of the presentinvention,

[0069]FIG. 6 is a block diagram showing an image processing apparatusfor carrying out an image processing method in accordance with a thirdembodiment of the present invention,

[0070]FIG. 7 is a view for illustrating division into blocks, adesignated block and reference blocks in the method of the thirdembodiment of the present invention,

[0071]FIG. 8A is a view for illustrating a conventional method of imageprocessing, and

[0072]FIG. 8B is a view for illustrating the drawbacks of theconventional method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0073]FIG. 1 shows an image processing apparatus for carrying out animage processing method in accordance with a first or second embodimentof the present invention. The image processing apparatus comprises animage input means 10, a brightness/chromaticity calculating means 20, abrightness/chromaticity memory means 30, a color border enhancementprocessing system 40 formed of a color border determining means 41, anrepresentative color determining means 42, a color border enhancingbrightness calculating means 43 and a switch 44, a color borderenhancing brightness memory means 50, a binary-coding means 60, and animage output means 70.

[0074] The color input means 10 reads a color original and samples RGBvalues of each pixel (8 bits for each color, 24 bits in total) . Thecolor input means 10 may comprise, for instance, an image scanner.

[0075] The brightness/chromaticity calculating means 20 converts RGBvalues of each pixel into L*a*b* values of CIE color system(L*:brightness, a*, b*:chromaticity). Conversion of RGB values intoL*a*b* values of CIE color system is described, for instance, in “ColorEngineering” by Noboru Ohta. The brightness/chromaticity memory means 30stores L*a*b* values of each pixel calculated by thebrightness/chromaticity calculating means 20.

[0076] The color border determining means 41 determines whether adesignated pixel P_(o) is a “color border pixel”, that is, whether thedesignated pixel P_(o) is on a border between two different colorregions, on the basis of L*a*b* values of the designated pixel P_(o)(L*_(Po,)a*_(Po,)b*_(Po)) and those of reference pixels Pi (i stands fora pixel number) near the designated pixel P_(o)(L*_(Pi,)a*_(Pi,)b*_(Pi)) The reference pixels will be sometimes called“neighbor pixels”. When the image processing apparatus carries out animage processing method in accordance with a first embodiment of thepresent invention, the color border determining means 41 sets 8 neighborpixels P_(i) (i=1 to 8) about each designated pixel P_(o) (3×3 pixelswith the designated pixel P_(o) included) as shown in FIG. 2A, anddetermines whether the designated pixel P_(o) is a “color border pixel”in the following manner. Change in brightness along a transverse lineX-X′ is as shown in the lower part of FIG. 2A.

[0077] a) brightness differences ΔL_(PoPi) (=ΔL_(xy)) and chromaticitydifferences ΔC_(PoPi) (=ΔC_(xy)) on a a*-b* plane between the designatedpixel P_(o) and the reference pixels P_(i) are calculated according tothe following formulae (1) and (2).

ΔL _(xy) L* _(x) −L* _(y)  (1

ΔC _(xy)={(a* _(x) −a* _(y))²+(b* _(x) −b* _(y))²}^(½)  (2)

[0078] b) When the brightness difference ΔL_(PoPi) between thedesignated pixel P_(o) and one of the reference pixels P_(i) and thechromaticity difference ΔC_(PoPi) between the designated pixel P_(o) andthe reference pixel P_(i) simultaneously satisfy the followingconditions (1) and (2), the reference pixel P_(i) is determined to be aprospective pixel of an opposed color which is in prospect of being apixel opposed to the designated pixel P_(o) on opposite sides of a colorborder (such a pixel will be referred to as “a prospective opposed colorpixel”, hereinbelow).

ΔL_(PoPi)≦Th₁  (condition 1)

ΔC_(PoPi)≧Th₂  (condition 2)

[0079] c) Steps a) and b) are executed for all the reference pixels P₁to P₈ and when at least one of the reference pixels is determined to bea prospective opposed color pixel, the designated pixel P_(o) isdetermined to be a color border pixel.

[0080] The representative color determining means 42 determines,according to a predetermined condition, the values of brightness andchromaticity of at least two colors representative of colors forming thecolor border for the designated pixel P_(o) determined to be a colorborder pixel.

[0081] The color border enhancing brightness calculating means 43calculates a color border enhancing brightness L*_(Po(new)) for thecolor border pixel P_(o) according to the L*a*b* values of the colorborder pixel P_(o) stored in the brightness/chromaticity memory means30, the L*a*b* values of the representative colors, and a preset orderof priority of colors.

[0082] The color border enhancing brightness memory means 50 stores thevalue of the brightness L*_(Po) which has been stored in thebrightness/chromaticity memory means 30 as it is for the designatedpixels L_(Po) which are not determined to be a color border pixel by thecolor border determining means 41 and stores the value of the colorborder enhancing brightness L*_(Po(new)) for the designated pixelsL_(Po) which are determined to be a color border pixel by the colorborder determining means 41, thereby obtaining a monochromatic imagesignal.

[0083] The switch 44 of the color border enhancement processing system40 selectively connects the brightness/chromaticity memory means 30 tothe color border enhancing brightness memory means 50 or therepresentative color determining means 42 so that the L*a*b* valuesstored in the brightness/chromaticity memory means 30 are input into therepresentative color determining means 42 (and then into the colorborder enhancing brightness calculating means 43) only when thedesignated pixel P_(o) is determined to be a color border pixel, and thevalue of the colorborder enhancing brightness L*_(Po(new)) for thedesignated pixels L_(Po) is input into the color border enhancingbrightness memory means 50, and so that the value of the brightnessL*_(Po) which has been stored in the brightness/chromaticity memorymeans 30 as it is input into the colorborder enhancing brightness memorymeans 50 when the designated pixel L_(Po) is not determined to be acolor border pixel by the color border determining means 41.

[0084] The binary-coding means 60 converts the monochromatic imagesignal stored in the color border enhancing brightness memory means 50into a two-valued (binary-coded) image signal.

[0085] The image output means 70 outputs a monochromatic image on thebasis of the binary-coded monochromatic image signal output from thebinary-coding means 60.

[0086] An image processing method in accordance with a first embodimentof the present invention as carried out by the image processing means 1will be described, hereinbelow.

[0087] The color input means 10 reads a color original and samples RGBvalues of each pixel. The brightness/chromaticity calculating means 20converts RGB values of each pixel into L*a*b* values of CIE color systemand the brightness/chromaticity memory means 30 stores L*a*b* values ofeach pixel calculated by the brightness/chromaticity calculating means20.

[0088] The color border determining means 41 determines whether adesignated pixel P_(o) is a color border pixel on the basis of L*a*b*values of the designated pixel P_(o) (L*_(Po,)a*_(Po,)b*_(Po)) and thoseof reference pixels Pi in the manner described above. When the pixeldesignated pixel P_(o) is determined to be a color border pixel, therepresentative color determining means 42 determines, according to apredetermined condition, the values of brightness and chromaticity of atleast two colors representative of colors forming the color border forthe designated pixel P_(o) determined to be a color border pixel. Thenthe color border enhancing brightness calculating means 43 calculates acolor border enhancing brightness L*_(Po(new)) for the color borderpixel P_(o) according to the L*a*b* values of the color border pixelP_(o) stored in the brightness/chromaticity memory means 30, the L*a*b*values of the representative colors, and a preset order of priority ofcolors.

[0089] Operation of the color border enhancement processing system 40will be described in more detail assuming that fifth and eighthreference pixels P₅ and P₈ are determined to be a prospective opposedcolor pixel for the designated pixel P_(o) in FIG. 2A. FIG. 4 shows thea*-b* plane for determining the representative colors.

[0090] a) The chromaticity difference ΔC_(P5Pi) (i=1 to 4, 6 to 8)between the fifth reference pixel P₅ and each of the other referencepixels P_(i) (i=1 to 4, 6 to 8) is calculated according to a formulasimilar to Formula (2) and the chromaticity difference ΔC_(P8Pi) (i=1 to7) between the eighth reference pixel P₈ and each of the other referencepixels P_(i) (i=1 to 7) is calculated according to a formula similar toFormula (2).

[0091] b) A maximum value Max{ΔC_(P5Pi), ΔC_(P8Pi)} in the chromaticitydifferences ΔC_(P5Pi) and ΔC_(P8Pi) are obtained.

[0092] c) For example, when a pair of reference pixels which give amaximum chromaticity difference are the first reference pixel P₁ and theeighth pixel P₈, that is, Max{ΔC_(P5Pi), ΔC_(P8Pi)}=ΔC_(P8P1), theL*a*b* values of the first and eighth reference pixels P₁ and P₈ storedin the brightness/chromaticity memory means 30, (L*_(P1), a*_(P1),b*_(P1)) and (L*_(P8), a*_(P8), b*_(P8)), are respectively taken as theL*a*b* values of the representative colors.

[0093] d) The L*a*b* values of the respective representative colors,(L*_(P1), a*_(P1), b*_(P1)) and (L*_(P8), a*_(P8), b*_(P8)), areprojected onto an a*-b* plane such as shown in FIG. 4, and it isdetermined which of reference colors black (K), white (W), cyan (C),magenta (M), yellow (Y), red (R), green (G), blue (B), each of therepresentative colors belongs to. As for black and white, when the valueL*_(P8) or L*_(P1) of brightness satisfies the following condition 3,the representative color is determined to be black while when the valueL*_(P8) or L*_(P1) of brightness satisfies the following condition 4,the representative color is determined to be white.

L*≦Th₃  (condition 3)

L*>Th₃  (condition 4)

[0094] wherein Th3 is a third threshold value.

[0095] e) Also as for designated pixel, it is determined which ofreference colors black (K), white (W), cyan (C), magenta (M), yellow(Y), red (R), green (G), blue (B), the color of the designated pixelP_(o) belongs to on the basis of the L*a*b* values of the designatedpixel, (L*_(Po), a*_(Po), b*_(Po)) , stored in thebrightness/chromaticity memory means 30.

[0096] f) Then it is determined which of the representative colors, thecolor of the designated pixel P_(o) belongs to in terms of the referencecolor. For example, when one of the representative colors is red (R) andthe other is yellow (Y) in terms of the reference color while the colorof the designated pixel P_(o) is red (R) in terms of the referencecolor, then the color of the designated pixel P_(o) is determined tobelong said one of the representative colors and when the color of thedesignated pixel P_(o) is yellow (Y) in terms of the reference color,then the color of the designated pixel P_(o) is determined to belongsaid the other of the representative colors.

[0097] When the color of the designated pixel P_(o) belongs to neitherof the representative colors in terms of the reference color, it isdetermined that the color of the designated pixel P_(o) belongs to oneof the representative colors which is smaller in the chromaticitydifference from the color of the designated pixel P_(o)(ΔC_(PoP1),ΔC_(PoP8)) than the other.

[0098] g) Then a color border enhancing brightness L*_(Po(new)) for thedesignated pixel P_(o) is calculated according to the following formula(3) on the basis of the L*a*b* values of the designated pixel P_(o)stored in the brightness/chromaticity memory means 30 and a preset orderof priority of colors shown in the following table 1 and stored in amemory (not shown), and the calculated color border enhancing brightnessL*_(Po(new)) for the designated pixel P_(o) is stored in the colorborder enhancing memory means 50. TABLE 1 order 1 2 3 4 5 6 7 8 color WY C M R G B K

[0099] (formula 3)

[0100] When the representative color which the designated pixel belongsto is lower in priority than the other representative color.

L* _(Po(new)) =L* _(Po)−α (α is a constant)

[0101] When the representative color which the designated pixel belongsto is higher in priority than the other representative color.

L* _(Po(new)) =L* _(Po)+α (α is a constant)

[0102] For example, when the representative colors are R and Y and thecolor of the designated pixel is R, L*_(Po(new))=L*_(Po)−α since R islower in priority than Y. Conversely, when the representative colors areR and Y and the color of the designated pixel is Y,L*_(Po(new))=L*_(Po)+α since Y is higher in priority than R.

[0103] As for the pixels P_(o) which are not determined to be a colorborder pixel by the color border determining means 41, the values ofbrightness L*_(Po) stored in the brightness/chromaticity memory means 30are stored as they are in the color border enhancing brightness memorymeans 50.

[0104] That is, for the pixels in the color border region, the values ofbrightness L*_(Po(new)) calculated by the color border enhancingbrightness calculating means 43 are stored in the color border enhancingbrightness memory means 50 whereas, for the pixels in the regions otherthan the color border region, the values of brightness L*_(Po) asrepresented by the color image signal are stored in the color borderenhancing brightness memory means 50.

[0105] That is, the color border enhancement processing system 40determines whether a designated pixel is a color border pixel in a colorborder region of a type where two color regions which are small indifference of brightness L* and large in difference of chromaticity (a*,b*) are positioned side by side. When it is determined that thedesignated pixel is a color border pixel, the color enhancementprocessing system 40 changes the value of brightness L* while when it isnot determined that the designated pixel is a color border pixel, thecolor enhancement processing system 40 uses the value of brightness L*of the pixel as represented by the color image signal. The colorenhancement processing system 40 repeats these steps while shifting thedesignated pixel one by one and changes the value of brightness L* ofthe color border pixel so that the pixels on one side of the colorborder becomes higher in value of brightness L* than the pixels on theother side of the color border. In this manner, the color enhancementprocessing system 40 generates a monochromatic image signal of oneframe.

[0106]FIG. 2B shows a monochromatic image obtained by carrying out thecolor border enhancement processing on the pixels of the color imageshown in FIG. 2A up to the designated pixel P_(o) with change inbrightness along a transverse line X-X′ shown in the lower part of FIG.2B.

[0107]FIG. 3 shows a monochromatic image obtained by carrying out thecolor border enhancement processing on all the pixels of the color imageshown in FIG. 2A with change in brightness along a transverse line X-X′shown in the lower part of FIG. 3.

[0108] Though, in the embodiment described above, the color border isenhanced by adding or subtracting a constant α to or from the value ofbrightness of the color border pixel according to formula (3), the valueof the constant α may be changed pixel by pixel, for instance, accordingto the value of chromaticity (a*, b*).

[0109] After the processing by the color enhancement processing system40 is ended, the binary-coding means 60 binary-codes the value ofbrightness for each pixel (L*_(Po(new)) or L*_(Po)) and the image outputmeans 70 outputs a monochromatic image on the basis of the binary-codedmonochromatic image signal output by the binary-coding means 60.

[0110] since the monochromatic image signal is obtained by changing thevalues of brightness of pixels in the color border region from thoserepresented by the color image signal so that the pixels on one side ofthe color border becomes higher in value of brightness L* than thepixels on the other side of the color border while holding the values ofbrightness of pixels in regions other than the color border region asrepresented by the color image signal, the colorborder region can beenhanced so as to be able to be recognized as change in tone in themonochromatic image while tone information on the region other than thecolor border region on the color image can be held even in themonochromatic image.

[0111] Accordingly, the problem that the regions which are differentfrom each other in both hue and brightness on the color image areexpressed in the same tone on the monochromatic image can be avoided,and information on change in hue on the color image can be expressed asinformation on brightness on the monochromatic image. Thus a color imagecan be converted into a monochromatic image without loss of informationon brightness and hue irrespective of brightness and hue on the colorimage.

[0112] An image processing method in accordance with a second embodimentof the present invention as carried out by the image processing means 1will be described, hereinbelow. The method of the second embodiment isbasically the same as the method of the first embodiment except theoperation of the color border enhancement processing section 40. Thatis, in the method of the first embodiment of the present invention, thecolor border enhancement processing is carried out only on the colorborder pixels. Whereas, in the second embodiment, the color borderenhancement processing is carried out on all the pixels in the blockabout a color border pixel.

[0113] Result of the method in accordance with the first embodiment inwhich color border enhancement processing is carried out only on thecolor border pixels is clear when the image input means 10 and the imageoutput means 70 are relatively low in resolution. However, when theimage input means 10 and the image output means 70 are relatively highin resolution (high in pixel density) , the effect of enhancement of thecolor border region by the method of the first embodiment becomesdifficult to recognize since the area of the enhanced region is small.In such a case, it is preferred that the color border enhancement becarried out over a certain area. In the method of the second embodiment,when a designated pixel P_(o) is determined to be a color border pixel,the color border enhancement processing is carried out on all the pixelsin a predetermined region about the designated pixel P_(o) as shown inFIG. 5A. The predetermined region is formed of the designated pixelP_(o) and 5×5 neighbor pixels P_(i) (i=1 to 24) about the designatedpixel P_(o) as shown in FIG. 5A, where the neighbor pixels are denotedby only the suffixes. Change in brightness along a transverse line X-X′is as shown in the lower part of FIG. 5A. Operation of the method of thesecond embodiment will be described, hereinbelow.

[0114] The processing up to the step d) of determining which ofreference colors black (K), white (W), cyan (C), magenta (M), yellow(Y), red (R), green (G), blue (B), each of the representative colorsbelongs to is the same as in the first embodiment.

[0115] After the step d), the following steps are executed.

[0116] e) The L*a*b* values of the designated pixel P_(o) and the 24neighbor pixels P_(i), (L*_(Po), a*_(Po), b*_(Po)) and (L*_(Pi),a*_(Pi), b*_(Pi)), are projected onto an a*-b* plane such as shown inFIG. 4, and it is determined which of reference colors black (K) , white(W), cyan (C), magenta (M), yellow (Y), red (R), green (G), blue (B),the color of each of the pixels belongs to. As for black and white, whenthe value L*_(Po) or L*_(Pi) of brightness satisfies the aforesaidcondition 3, the color of the pixel is determined to be black while whenthe value L*_(Po) or L*_(Pi) of brightness satisfies the aforesaidcondition 4, the color of the pixel is determined to be white.

[0117] f) Then it is determined which of the representative colors, thecolor of the designated pixel P_(o) and the colors of the 24 neighborpixels P_(i) belong to in terms of the reference color in the mannersimilar to that in the first embodiment.

[0118] g) Then a color border enhancing brightness L*_(Po(new)) for thedesignated pixel P_(o) is calculated according to the aforesaid formula(3) on the basis of the L*a*b* values of the designated pixel P_(o)stored in the brightness/chromaticity memory means 30 and a preset orderof priority of colors shown in the aforesaid table 1, and the calculatedcolor border enhancing brightness L*_(Po(new)) for the designated pixelP_(o) is stored in the color border enhancing memory means 50. In thesimilar manner, a colorborder enhancing brightness L*_(Pi(new)) for eachof the neighbor pixels P_(i) is calculated according to the aforesaidformula (3) on the basis of the L*a*b* values of the pixel P_(i) storedin the brightness/chromaticity memory means 30 and a preset order ofpriority of colors shown in the aforesaid table 1, and the calculatedcolor border enhancing brightness L*_(Pi(new)) for the pixel P_(i) isstored in the color border enhancing memory means 50. The pixels whichhave been subjected to the color border enhancement processing inconjunction with a preceding designated pixel P_(o) are not subjected tothe color border enhancement processing at this time.

[0119] As for the pixels P_(o) which are not determined to be a colorborder pixel by the color border determining means 41, the values ofbrightness L*_(Po) stored in the brightness/chromaticity memory means 30are stored as they are in the color border enhancing brightness memorymeans 50.

[0120] That is, for the pixels in the color border region, the values ofbrightness L*_(Po(new)) calculated by the color border enhancingbrightness calculating means 43 are stored in the color border enhancingbrightness memory means 50 whereas, for the pixels in the regions otherthan the color border region, the values of brightness L*_(Po) asrepresented by the color image signal are stored in the colorborderenhancing brightness memory means 50.

[0121] That is, the color border enhancement processing system 40determines whether a designated pixel is a color border pixel in a colorborder region of a type where two color regions which are small indifference of brightness L* and large in difference of chromaticity (a*,b*) are positioned side by side. When it is determined that thedesignated pixel is a color border pixel, the color enhancementprocessing system 40 changes the values of brightness L* of thedesignated pixel P_(o) and the 24 neighbor pixels P_(i) while when it isnot determined that the designated pixel is a color border pixel, thecolor enhancement processing system 40 uses the value of brightness L*of the pixel as represented by the color image signal. The colorenhancement processing system 40 repeats these steps while shifting thedesignated pixel one by one and changes the values of brightness L* ofthe color border pixel and its neighbor pixels. In this manner, thecolor enhancement processing system 40 generates a monochromatic imagesignal of one frame.

[0122]FIG. 5B is a monochromatic image obtained by carrying out thecolor border enhancement processing in accordance with the secondembodiment on all the pixels of the color image shown in FIG. 5A withchange in brightness along a transverse line X-X′ shown in the lowerpart of FIG. 5B.

[0123] After the processing by the color enhancement processing system40 is ended, the binary-coding means 60 binary-codes the value ofbrightness for each pixel (L*_(Po(new)) or L*_(Po)) and the image outputmeans 70 outputs a monochromatic image on the basis of the binary-codedmonochromatic image signal output by the binary-coding means 60.

[0124] Since the monochromatic image signal is obtained by changing thevalues of brightness of pixels in the color border region from thoserepresented by the color image signal while holding the values ofbrightness of pixels in regions other than the color border region asrepresented by the color image signal, the color border region can beenhanced so as to be recognized as change in tone in the monochromaticimage while tone information on the region other than the color borderregion on the color image can be held even in the monochromatic image.

[0125] Further, in accordance with the method of the second embodiment,since the color border is enhanced over a certain width (in the range of5×5 pixels in the particular embodiment described above), the area ofenhanced region is increased and color border enhancement can be easilyrecognized even when the image input means 10 and the image output means70 are high in resolution.

[0126] Further, though, in the embodiment described above, the pixelswhich have been subjected to the color border enhancement processing inconjunction with a preceding designated pixel P_(o) are not subjected tothe color border enhancement processing. However, it is possible tocarry out the colorborder enhancement processing every time the pixel isset as a designated pixel or the neighbor pixel. For example, the colorborder enhancing brightness L*_(Pi(new)) calculated before may bereplaced by the newly calculated color border enhancing brightnessL*_(Pi(new)), or one of the color border enhancing brightnessL*_(Pi(new)) calculated before and the newly calculated color borderenhancing brightness L*_(Pi(new)) may be selected under a certaincondition, or the average of the values may be taken as a final colorborder enhancing brightness L*_(Pi(new)) for the pixel. That is, thoughthe color border enhancing brightness L*_(Pi(new)) may be calculated invarious manners, it is preferred that the color border enhancingbrightness L*_(Pi(new)) be calculated so that an excessive enhancementis not caused.

[0127] An image processing method in accordance with a third embodimentof the present invention will be described, hereinbelow. The imageprocessing method of this embodiment is carried out, for instance, bythe image processing apparatus shown in FIG. 6. The image processingapparatus shown in FIG. 6 is basically the same as that shown in FIG. 1except that the image is divided into a plurality of blocks of apredetermined size, whether the block is in a colorborder region isdetermined and each of the pixels in the block determined to be a colorborder block is subjected to the color border enhancement processing inthe third embodiment whereas whether the pixel is a color border pixelis determined pixel by pixel and the color border enhancement processingis carried out pixel by pixel in the first embodiment. Mainly thedifference of the third embodiment from the first embodiment will bedescribed, hereinbelow.

[0128]FIG. 6 shows an image processing apparatus for carrying out animage processing method in accordance with a third embodiment of thepresent invention. As shown in FIG. 6, the color border enhancementprocessing system 40 in this embodiment comprises a blockbrightness/chromaticity calculating means 46 and a blockbrightness/chromaticity memory means 47 in addition to the color borderdetermining means 41, the representative color determining means 42, thecolor border enhancing brightness calculating means 43 and the switch 44in the color border enhancement processing system 40 in the firstembodiment.

[0129] When the color image signal represents an image such as of acolor halftone original formed by superposing three colors, C, M and Y,where color frequently changes depending on the place, there is fearthat C, M and Y are independently read by the image input means 10 and acolor border other than that visually recognized by humans can bedetected if the color border region is determined pixel by pixel, whichcan result in undesirable brightness enhancement. In such a case, bydetermining the color border region block by block as in the method ofthe second aspect of the present invention, the color border region canbe determined and brightness can be enhanced so as to conform to humanview.

[0130] In the third embodiment, a monochromatic image is made from acolor image in the following manner. The steps from the step of readingthe color original by the image input means 10 to the step of storingthe L*a*b* values in the brightness/chromaticity memory means 30 arebasically the same as those in the first embodiment.

[0131] In the third embodiment, the following steps a) and b) areexecuted after the step of storing the L*a*b* values in thebrightness/chromaticity memory means 30 and before the processing by thecolor border enhancement processing system 40.

[0132] a) The color image signal representing the L*a*b* values of eachpixel is divided into a plurality of blocks of a predetermined size bythe block brightness/chromaticity calculating means 46. It is preferredthat the size of each block be larger than the size of the range of theneighbor pixels (3×3) which is used for the color border enhancement. Inthis particular embodiment, the size of each block is set to be 6×6 asshown in FIG. 7.

[0133] b) Then, the average of the L*a*b* values of the pixels in eachblock and the average is taken as the block L*a*b* values for the block.The block L*a*b* values are stored in the block brightness/chromaticitymemory means 47.

[0134] Then in the color border enhancement processing system 40, theprocessing basically the same as that in the first embodiment is carriedout. That is, the color border determining means 41 determines whetherthe designated block B_(o) is a color border block on the basis of theL*a*b* values of the designated block and those of the reference blocksB_(i) stored in the block brightness/chromaticity memory means 47. Whenthe designated block B_(o) is determined to be a color border block, thevalues of brightness and chromaticity of at least two colorsrepresentative of colors forming the color border are determined by therepresentative color determining means 42 in the same manner as in thefirst embodiment.

[0135] The color border enhancing brightness calculating means 43calculates a color border enhancing brightness L*_(Po(new)) for eachpixel in the color border block B_(o) according to the L*a*b* values ofthe pixel stored in the brightness/chromaticity memory means 30, theL*a*b* values of the representative colors, and a preset order ofpriority of colors in the following manner.

[0136] c) The color border enhancing brightness calculating means 43designates one of the pixels in the color border block B_(o) andcalculates the averages of the L*a*b* values of the designated pixelP_(o) and its 8 neighbor pixels P_(i) stored in thebrightness/chromaticity memory means 30. Then the color border enhancingbrightness calculating means 43 takes averages as the L*a*b* values ofthe designated pixel.

[0137] d) The L*a*b* values of the designated pixel P_(o) thus obtainedare projected onto an a*-b* plane such as shown in FIG. 4, and it isdetermined which of reference colors black (K), white (W), cyan (C),magenta (M), yellow (Y), red (R), green (G), blue (B), the color of thedesignated pixel P_(o) belongs to. As for black and white, when thevalue L*_(Po) of brightness satisfies the aforesaid condition 3, thecolor of the pixel is determined to be black while when the valueL*_(Po) of brightness satisfies the aforesaid condition 4, the color ofthe pixel is determined to be white. In this embodiment, since theaverages of the L*a*b* values of 9 pixels are taken as the L*a*b* valuesof the designated pixel P_(o), color determination is effected on thebasis of the L*a*b* values of a certain area. Accordingly, even if thecolor image signal represents an image such as of a color halftoneoriginal where color frequently changes depending on the place, thecolor border region can be determined more accurately so as to conformto human view as compared with when the color determination is effectedpixel by pixel.

[0138] e) Then it is determined which of the representative colors, thecolor of the designated pixel P_(o) belongs to in terms of the referencecolor in the manner similar to that in the first embodiment.

[0139] f) Then a color border enhancing brightness L*_(Po(new)) for thedesignated pixel P_(o) is calculated according to the aforesaid formula(3) on the basis of the L*a*b* values of the designated pixel P_(o)stored in the brightness/chromaticity memory means 30 and a preset orderof priority of colors shown in the aforesaid table 1, and the calculatedcolor border enhancing brightness L*_(Po(new)) for the designated pixelP_(o) is stored in the color border enhancing memory means 50. In thesimilar manner, a color border enhancing brightness for each of thepixels in the color border block is calculated.

[0140] Block B₅ in FIG. 7 is designated next, and the same steps arerepeated on the block B₅ using a reference region of 3×3 size. In thismanner, effecting the color border enhancement on the same pixel two ormore times can be prevented, whereby fear of excessively enhancing thecolor border can be avoided.

[0141] As can be understood from the description above, in accordancewith the third embodiment, even if the input color image signalrepresents an image such as of a color halftone original formed bysuperposing three colors, C, M and Y, the color border region can bedetermined so as to conform to human view. This is also true for a colorimage obtained from a silver halide picture, where color frequentlychanges depending on the place. Further, as in the second embodiment,since the color border is enhanced over a certain width (in the range of6×6 pixels in the particular embodiment described above), the area ofenhanced region is increased and color border enhancement can be easilyrecognized even when the image input means 10 and the image output means70 are high in resolution.

[0142] The present invention need not be limited to the embodimentsdescribed above.

[0143] For example, in place of L*a*b* values of CIE color system,L*u*v* values of CIE color system may be used. Further, thebrightness/chromaticity calculating means 20 maybe eliminated so thatthe RGB values from the image input means 10 are directly used.

[0144] Though, in the embodiment described above, the image input means10 outputs color information on the original as RGB values, the imageinput means 10 may output color information on the original as othervalues.

[0145] Further, though, in the embodiment described above, the colorborder enhancing brightness calculating means 43 increases values ofbrightness of pixels on one side of the color border while reducingvalues of brightness of pixels on the other side of the color border,the color border enhancing brightness calculating means 43 may enhancethe color border in various ways. For example, it may increase values ofbrightness of pixels on one side of the color border with values ofbrightness of pixels on the other side of the color border left as theyare.

[0146] The image processing method of the present invention may be usedfor displaying a color image on a monochromatic display, or may be usedfor transferring a color original from a color facsimile to a black andwhite facsimile.

What is claimed is:
 1. An image processing method of making amonochromatic image signal made up of image signal componentsrepresenting image data on respective pixels from a color image signalmade up of image signal components representing image data on respectivepixels, the method characterized in that values of brightness of pixelsin a color border region of a type where two color regions which aresmall in brightness difference and large in chromaticity difference arepositioned side by side are changed to new values different from thoserepresented by the image signal components for the pixels of the colorimage signal, and a monochromatic image signal is generated with valuesof brightness of pixels in the color border region set to said newvalues and values of brightness of pixels in regions other than thecolor border region set to those as represented by the image signalcomponents for the pixels of the color image signal.
 2. An imageprocessing method of making a monochromatic image signal made up ofimage signal components representing image data on respective pixelsfrom a color image signal made up of image signal componentsrepresenting image data on respective pixels, the method comprising thestep of determining whether a designated pixel is a pixel in a colorborder region of a type where two color regions which are small inbrightness difference and large in chromaticity difference arepositioned side by side on the basis of the difference in value ofbrightness between the designated pixel and neighbor pixels near thedesignated pixel and the difference in value of chromaticity between thedesignated pixel and the neighbor pixels, the steps of, when thedesignated pixel is determined to be a pixel in a color border region ofsaid type, obtaining the values of brightness and chromaticity of atleast two colors representative of colors forming the color border onthe basis of the values of brightness and chromaticity of the neighborpixels, and changing the value of brightness of the designated pixel toa new value different from that represented by the image signalcomponent for the pixel of the color image signal on the basis of thevalues of brightness and chromaticity of the designated pixel asrepresented by the image signal component for the pixel of the colorimage signal, the values of brightness and chromaticity of therespective representative colors and a preset order of priority ofcolors, and the step of making a monochromatic image signal by settingvalues of brightness of pixels in the color border region to said newvalues and setting values of brightness of pixels in regions other thanthe color border region to those as represented by the image signalcomponents for the pixels of the color image signal.
 3. An imagingprocessing method as defined in claim 2 in which, when the designatedpixel is determined to be a pixel in a color border region of said type,the value of brightness of each pixel in a predetermined region aboutthe designated pixel is changed to a value different from thatrepresented by the image signal component for the pixel of the colorimage signal on the basis of the values of brightness and chromaticityof the pixel, the values of brightness and chromaticity of therespective representative colors and a preset order of priority ofcolors.
 4. An image processing method of making a monochromatic imagesignal made up of image signal components representing image data onrespective pixels from a color image signal made up of image signalcomponents representing image data on respective pixels, the methodcomprising the steps of dividing the color image signal into a pluralityof blocks of a predetermined size, and determining the values ofbrightness and chromaticity of each block on the basis of the values ofbrightness and chromaticity of each pixel in the block represented byimage signal components for the pixel of the color image signal, thestep of determining whether a designated block is a block in a colorborder region of a type where two color regions which are small inbrightness difference and large in chromaticity difference arepositioned side by side on the basis of the difference in value ofbrightness between the designated block and neighbor blocks near thedesignated block and the difference in value of chromaticity between thedesignated block and the neighbor blocks, the steps of, when thedesignated block is determined to be a block in a color border region ofsaid type, obtaining the values of brightness and chromaticity of atleast two colors representative of colors forming the color border onthe basis of the values of brightness and chromaticity of the neighborblocks, and changing the value of brightness of each pixel in thedesignated block to a new value different from that represented by theimage signal component for the pixel of the color image signal on thebasis of the values of brightness and chromaticity of the pixel, thevalues of brightness and chromaticity of the respective representativecolors and a preset order of priority of colors, and the step of makinga monochromatic image by setting values of brightness of pixels in theblock in the color border region to said new values and setting valuesof brightness of pixels in the blocks in regions other than the colorborder region to those as represented by the image signal components forthe pixels of the color image signal.
 5. An image processing apparatusfor making a monochromatic image signal made up of image signalcomponents representing image data on respective pixels from a colorimage signal made up of image signal components representing image dataon respective pixels, the apparatus characterized by having a colorborder enhancement processing means which changes values of brightnessof pixels in a color border region of a type where two color regionswhich are small in brightness difference and large in chromaticitydifference are positioned side by side to new values different fromthose represented by the image signal components for the pixels of thecolor image signal and a monochromatic image signal generating meanswhich makes a monochromatic image signal by setting values of brightnessof pixels in the color border region to said new values and settingvalues of brightness of pixels in regions other than the color borderregion to those as represented by the image signal components for thepixels of the color image signal.
 6. An image processing apparatus formaking a monochromatic image signal made up of image signal componentsrepresenting image data on respective pixels from a color image signalmade up of image signal components representing image data on respectivepixels comprising a color border determining means which determineswhether a designated pixel is a pixel in a color border region of a typewhere two color regions which are small in brightness difference andlarge in chromaticity difference are positioned side by side on thebasis of the difference in value of brightness between the designatedpixel and neighbor pixels near the designated pixel and the differencein value of chromaticity between the designated pixel and the neighborpixels, a representative color determining means which obtains thevalues of brightness and chromaticity of at least two colorsrepresentative of colors forming the color border concerning thedesignated pixel determined to be a pixel in the color border region onthe basis of the values of brightness and chromaticity of the neighborpixels, a color border enhancing brightness calculating means whichchanges the value of brightness of the designated pixel determined to bea pixel in the color border region to a new value different from thatrepresented by the image signal component for the pixel of the colorimage signal on the basis of the values of brightness and chromaticityof the designated pixel as represented by the image signal component forthe pixel of the color image signal, the values of brightness andchromaticity of the respective representative colors and a preset orderof priority of colors, and a monochromatic image signal generating meanswhich generates a monochromatic image signal by setting values ofbrightness of pixels in the color border region to said new values andsetting values of brightness of pixels in regions other than the colorborder region to those as represented by the image signal components forthe pixels of the color image signal.
 7. An image processing apparatusas defined in claim 6 in which the color border enhancing brightnesscalculating means changes the value of brightness of each pixel in apredetermined region about the designated pixel to a new value differentfrom that represented by the image signal component for the pixel of thecolor image signal on the basis of the values of brightness andchromaticity of the pixel, the values of brightness and chromaticity ofthe respective representative colors and a preset order of priority ofcolors when the designated pixel is determined to be a pixel in thecolor border region of said type, and the monochromatic image signaldetermining means generates a monochromatic image signal by settingvalues of brightness of pixels in the predetermined region to said newvalues.
 8. An image processing apparatus as defined in claim 6 in whichthe color border determining means determines that the designated pixelis a pixel in a color border region of said type when there is at leastone pixel in the neighbor pixels whose difference in brightness fromthat of the designated pixel is smaller than a first threshold value andwhose difference in chromaticity from that of the designated pixel islarger than a second threshold value.
 9. An image processing apparatusas defined in claim 6 in which the representative color determiningmeans calculates the differences in chromaticity between a pixel in theneighbor pixels whose difference in brightness from that of thedesignated pixel is smaller than a first threshold value and whosedifference in chromaticity from that of the designated pixel is largerthan a second threshold value and the other pixels in the neighborpixels, and takes as the values of chromaticity of the representativecolors the values of chromaticity of a pair of pixels which give amaximum difference in chromaticity.
 10. An image processing apparatus asdefined in claim 6 in which the color border enhancing brightnesscalculating means determines which of the representative colors thecolor of the designated pixel belongs to on the basis of the values ofbrightness and chromaticity of the designated pixel and the values ofbrightness and chromaticity of each of the representative colors,changes the value of brightness of the designated pixel to a new valueby adding a predetermined constant to that represented by the imagesignal component for the pixel of the color image signal when therepresentative color to which the color of the designated pixel belongsis higher in the order of priority of colors than the otherrepresentative color, and changes the value of brightness of thedesignated pixel to a new value by subtracting a predetermined constantfrom that represented by the image signal component for the pixel of thecolor image signal when the representative color to which the color ofthe designated pixel belongs is lower in the order of priority of colorsthan the other representative color.
 11. An image processing apparatusas defined in claim 10 in which the color border enhancing brightnesscalculating means determines which of the representative colors thecolor of each pixel in a predetermined region belongs to on the basis ofthe values of brightness and chromaticity of the pixel and the values ofbrightness and chromaticity of each of the representative colors,changes the value of brightness of the pixel to a new value by adding apredetermined constant to that represented by the image signal componentfor the pixel of the color image signal when the representative color towhich the color of the pixel belongs is higher in the order of priorityof colors than the other representative color, and changes the value ofbrightness of the designated pixel to a new value by subtracting apredetermined constant from that represented by the image signalcomponent for the pixel of the color image signal when therepresentative color to which the color of the pixel belongs is lower inthe order of priority of colors than the other representative color. 12.An image processing apparatus as defined in claim 6 in which the orderof priority of colors is in the order of white, yellow, cyan, magenta,red, green, blue, black or in the order of the values of brightness ofthe representative colors.
 13. An image processing apparatus for makinga monochromatic image signal made up of image signal componentsrepresenting image data on respective pixels from a color image signalmade up of image signal components representing image data on respectivepixels comprising a block brightness/chromaticity calculating meanswhich divides the color image signal into a plurality of blocks of apredetermined size and determines the values of brightness andchromaticity of each block on the basis of the values of brightness andchromaticity of each pixel in the block represented by image signalcomponents for the pixel of the color image signal, a color borderdetermining means which determines whether a designated block is a blockin a color border region of a type where two color regions which aresmall in brightness difference and large in chromaticity difference arepositioned side by side on the basis of the difference in value ofbrightness between the designated block and neighbor blocks near thedesignated block and the difference in value of chromaticity between thedesignated block and the neighbor blocks, a representative colordetermining means which obtains the values of brightness andchromaticity of at least two colors representative of colors forming thecolor border concerning the designated block determined to be a block inthe color border region on the basis of the values of brightness andchromaticity of the neighbor blocks, a color border enhancing brightnesscalculating means which changes the value of brightness of each of thepixels in the block determined to be a block in the color border regionto a new value different from that represented by the image signalcomponent for the pixel of the color image signal on the basis of thevalues of brightness and chromaticity of the pixel as represented by theimage signal component for the pixel of the color image signal, thevalues of brightness and chromaticity of the respective representativecolors and a preset order of priority of colors, and a monochromaticimage signal generating means which generates a monochromatic imagesignal by setting values of brightness of pixels in the color borderregion to said new values and setting values of brightness of pixels inregions other than the color border region to those as represented bythe image signal components for the pixels of the color image signal.14. An image processing apparatus as defined in claim 13 in which thecolor border determining means determines that the designated block is ablock in a color border region of said type when there is at least oneblock in the neighbor blocks whose difference in brightness from that ofthe designated block is smaller than a first threshold value and whosedifference in chromaticity from that of the designated block is largerthan a second threshold value.
 15. An image processing apparatus asdefined in claim 13 in which the representative color determining meanscalculates the differences in chromaticity between a block in theneighbor blocks whose difference in brightness from that of thedesignated block is smaller than a first threshold value and whosedifference in chromaticity from that of the designated pixel is largerthan a second threshold value and the other blocks in the neighborblocks, and takes as the values of chromaticity of the representativecolors the values of chromaticity of a pair of blocks which give amaximum difference in chromaticity.
 16. An image processing apparatus asdefined in claim 13 in which the color border enhancing brightnesscalculating means determines which of the representative colors thecolor of each pixel in the block determined to be a block in a colorborder region belongs to on the basis of the values of brightness andchromaticity of the pixel and the values of brightness and chromaticityof each of the representative colors, changes the value of brightness ofthe pixel to a new value by adding a predetermined constant to thatrepresented by the image signal component for the pixel of the colorimage signal when the representative color to which the color of thepixel belongs is higher in the order of priority of colors than theother representative color, and changes the value of brightness of thepixel to a new value by subtracting a predetermined constant from thatrepresented by the image signal component for the pixel of the colorimage signal when the representative color to which the color of thepixel belongs is lower in the order of priority of colors than the otherrepresentative color.
 17. An image processing apparatus as defined inclaim 13 in which the order of priority of colors is in the order ofwhite, yellow, cyan, magenta, red, green, blue, black or in the order ofthe values of brightness of the representative colors.